There is known a projector that uses, as a light source, a high-luminance white light source such as a discharge lamp. Recently, research and development have actively been pursued to apply light sources other than the discharge lamp for the projector.
A solid-state light source such as a LED or a laser attracts attention as a light source excellent in life and color reproducibility. Particularly, the laser has characteristics of linear beam advancement and a single wavelength. The laser can accordingly contribute, when used as a light source for the projector, to miniaturization and higher performance of the apparatus.
However, when the laser is used as the light source for the projector, a beam is highly coherent, and hence the reduction of speckle noise is needed.
JP2007-280793A, which is a technology related to the present invention, discloses a compact lighting apparatus capable of reducing speckles.
As illustrated in FIG. 1, the lighting apparatus includes a light source such as a laser, light guide 505, entrance-side reflecting unit 503, and exit-side reflecting unit 506. Light guide 505 is a member to receive light from the light source. Entrance-side reflecting unit 503 is disposed on an entrance-side end surface of light guide 505, and aperture 503a is formed therein. Exit-side reflecting unit 506 is disposed on an exit-side end surface of light guide 505, and aperture 506a is formed therein.
The light that has entered light guide 505 is repeatedly reflected on an inner surface in light guide 505 to exit from aperture 506a of the exit side. In other words, light guide 505 is similar in operation to a rod integrator. The angle of the light from the light source is once widened. The light whose angle is widened is condensed at aperture 503a of the entrance side of light guide 505 by condenser lens 102. The light that has exited from light guide 505 is parallelized by a lens system to illuminate an image display panel.
As described above, a light flux that has entered from aperture 503a of the entrance side of light guide 505 is repeatedly reflected on the inner surface in light guide 505 to advance, and exits from aperture 506a of the exit side. On the other hand, light unable to exit from aperture 506a of the exit side of light guide 505 is reflected by exit-side reflecting unit 506. The light reflected by exit-side reflecting unit 506 advances toward the entrance side of light guide 505, and is reflected by entrance-side reflecting unit 503 excluding aperture 503a of the entrance side. The light reflected by entrance-side reflecting unit 503 is changed again in the advancing direction to the exit side. The light that has reached the aperture of exit-side reflecting unit 506 can exit from light guide 505 to be used as illumination light.
By reciprocating the light from the light source between exit-side reflecting unit 506 and entrance-side reflecting unit 503, as described above, the phase distribution of the light flux exiting from light guide 505 is randomly changed. Even a light source such as a laser source where coherence is high can reduce interferences between rays. Thus, the lighting apparatus disclosed in JP2007-280793A can reduce speckles of a projected image as a projector.